The present invention relates to a remotely assemblable and disassemblable fuel assembly for a nuclear reactor and to the apparatus for performing said assembly and assembly operations for the said fuel assembly.
More specifically, the present invention relates to a fuel assembly for nuclear reactors of the water type, said fuel assembly comprising fuel elements in the form of rods fixed within a rigid structure.
It is known that in pressurized water reactors the reactor core comprises a certain number of juxtaposed fuel assemblies. In known manner, these fuel assemblies comprise mechanical structures permitting the joining of a certain number of fuel elements essentially constituted by fissile material. The present invention relates to the case where the fuel elements comprise fuel rods, i.e. elements whose active part is constituted by a fissile material having an elongated cylindrical shape placed within a protective can made for example from a thin layer of a metal such as zircaloy.
The fuel element can constitutes one of the barriers ensuring the operating safety of the reactor. More specifically, this barrier confines the gases and other fission products resulting from the neutron reaction with respect to the coolant circulating in the reactor vessel and which is constituted by water. It is clear that it is particularly important that this can retain its integrity to obviate the presence of fission products in the cooling water, when the latter leaves the reactor vessel. When an abnormal quantity of fission products is detected in the reactor cooling water, thus indicating an important fracture of the fuel element can, it is necessary to shut down the reactor to change the fuel assembly and replace it by a new fuel assembly.
However, it should be noted that a fuel assembly is very expensive due to the fact that the fuel elements are themselves expensive and due to the fact that the mechanical structure holding the fuel elements in place in the assembly is made from relatively expensive materials. It is therefore obvious that when a single fuel element in an assembly is defective that the greatest interest is attached to the possibility of disassembling the fuel assembly in order to change the defective fuel element. However, it is obvious that this disassembly cannot take place in a normal environment. Disassembly can for example take place in a hot cell ensuring insulation between the personnel carrying out the work and the fuel element, however, this operation is extremely difficult. Another solution consists of removing the fuel assembly from the reactor vessel and placing it in a handling pond in which the fuel element replacement operations are carried out.
It is clear that the second operating procedure offers the double advantage of on the one hand permitting only the replacement of the defective fuel element whilst retaining the remainder of the fuel assembly and on the other of permitting working in a relatively easy manner and in a relatively simple installation constituted by a pond. However, for this operation to be performable it is necessary for the fuel assembly to be constructed in such a way that it can be remotely disassembled.
Systems permitting the remote disassembly of fuel assemblies have already been proposed. This disassembly essentially relates to the removal of one of the end plates of the fuel assembly from the brace rods constituting the remainder of the mechanical structure of the fuel assembly. However, in the hitherto recommended solution, the disassemblable connection between one of the ends of the brace rods and one of the end plates of the fuel assembly has the disadvantage that certain of the assembly member components and more specifically the component which joins the brace rods to one of the end plates in the case of a fracture of the said assembly member become free and can therefore circulate in the reactor vessel and in adjacent installations, thereby impeding the circulation of the cooling liquid and causing material damage, particularly in the heat exchanger tubes. It is obvious that such assembly members are unacceptable.